gdns: Tools to Work with Google DNS Over HTTPS API

To address the problem of insecurity of UDP-based DNS requests,
Google Public DNS offers DNS resolution over an encrypted HTTPS
connection. DNS-over-HTTPS greatly enhances privacy and security
between a client and a recursive resolver, and complements DNSSEC
to provide end-to-end authenticated DNS lookups. Functions that enable
querying individual requests that bulk requests that return detailed
responses and bulk requests are both provided. Support for reverse
lookups is also provided. See <https://developers.google.com/speed/public-dns/docs/dns-over-https>
for more information.